Pickling acids, such as hydrochloric acid and nitric acid, are used in the treatment of iron and other metals. Typically, the spent acids are regenerated for reuse in various processes to minimize waste and problems associated with disposal.
Several acid regeneration processes pass the spent acid through a furnace to pyrolyze or roast the various components and to produce acid vapors. The vapors are passed through an absorption column counter-currently to an absorption liquid which is typically water. The regenerated acid is then withdrawn through the bottom of the column. The pyrolysis exhaust gases are withdrawn from the head space of the absorption column by an external exhaust fan which is supplied with a water spray to separate particulates remaining in the exhaust gas steam before discharging through a chimney.
Pickling acids of hydrochloric acid used to treat iron or steel produce numerous metal chlorides in the spent acid liquor. Many metal chlorides can be thermally converted into metal oxides and hydrogen chloride. The metal oxides then can be separated from the gaseous hydrogen chloride which can be recovered as hydrochloric acid for regenerating the pickling acids.
Typical processes of regenerating spent acids spray the aqueous acid solution into a heated reactor. The heated reactor heats the acid solution to vaporize the aqueous phase and acid compounds and produce particulates of metal salts that are contained in the spent acids as impurities and waste byproducts. The droplets of the spent acid solution and the particulates of the metal salts fall downwardly through the reactor where the particulates are removed. This process results in vary short retention time in the reactor, usually only a few seconds, which often produces metal oxide particulates having acid residues on the surface of the metal oxide particulates. The acid residues contaminate the metal oxides, thereby requiring further processing before the metal oxides can be used. The reactor temperature can be increased to completely separate the acid residues from the metal oxide particulates. Increasing the reactor temperature often overheats and decomposes a portion of the metal oxide particulates. Furthermore, the acid liquor is usually injected into the middle of the reactor so that the droplets in the middle are insufficiently heated, while the droplets on the outer areas of the reactor are overheated and partially decompose. In addition, increasing the reactor temperature results in a time lag before the desired roasting conditions are attained which can result in large amounts of acid residues being discharged with the metal oxides particulates. Other examples of processes for regenerating acids are disclosed in U.S. Pat. No. 4,049,788 and U.S. Pat. No. 5,149,515.
Accordingly, a continuing need exists in the industry for an improved process for the roasting of spent acid liquors.